Handover during secondary cell group failure

ABSTRACT

A user device operating in dual-mode connectivity with a master node and a secondary node detects a failure of a secondary cell group associated with the secondary node ( 1104 ). In response, the user device suspends an secondary cell group transmission ( 1106 ). The user device generates a message related to a status of the SCG for transmission to the MN ( 1108 ) and transmitting the message to the MN when the SCG transmission is suspended ( 1110 ).

FIELD OF THE DISCLOSURE

This disclosure relates generally to wireless communications and, moreparticularly, to handling handover in dual connectivity (DC) when a cellgroup associated with the secondary node fails.

BACKGROUND

A user device (or user equipment, commonly denoted by acronym “UE”) insome cases can operate in DC with a master node (MN) and a secondarynode (SN). The MN can provide a control plane connection and a userplane connection to a core network (CN), whereas the SN generallyprovides a user plane connection. The cells associated with the MNdefine a master cell group (MCG), and the cells associated with the SNdefine a secondary cell group (SCG).

The UE and the base stations MN and SN can use signaling radio bearers(SRBs) to exchange radio link control (RRC) messages, as well asnon-access stratum (NAS) messages. There are several types of SRBs thatUEs can use when operating in DC, known as SRB0, SRB1, etc. resources.SRB0 resources support RRC messages over the common control channel(CCCH); SRB1 resources carry RRC messages, which in some cases includenon-access stratum (NAS) messages over the dedicated control channel(DCCH); SRB2 resources support RRC messages that include loggedmeasurement information or NAS messages, also over the DCCH but withlower priority than SRB1 resources; SRB3 resources support RRC messagesrelated to measurement configuration and reporting, for example, of anSCG when the UE operates in DC; and SRB4 resources support RRC messagesthat include application layer measurement reporting information, overthe DCCH.

Generally speaking, SRB1 and SRB2 resources allow the UE and the MN toexchange RRC messages related to the MN, and to embed RRC messagesrelated to the SN, and can be referred to as MCG SRBs. SRB3 resourcesallow the UE and the SN to exchange RRC messages related to the SN, andcan be referred to as SCG SRBs. Further, split SRBs allow the UE toexchange RRC messages directly with the MN and the SN. Still further,the UE and the base stations MN and SN use data radio bearers (DRBs) totransport data on a user plane. DRBs that terminate at the MN and uselower-layer resources of only the MN can be referred as MCG DRBs, DRBsthat terminate at the SN and use lower-layer resources of only the SNcan be referred as SCG DRBs, and DRBs that terminate at either the MCGor the SCG but use lower-layer resources of both the MN and the SN canbe referred to as split DRBs.

After an MN provides a UE with SCG configuration, the UE can detect SCGfailure (which can be a radio link failure (RLF), for example). In somescenarios, the SCG failure prevents the MN from coordinating a handoverprocedure for the UE, when a better signal quality or signal strength isavailable.

SUMMARY

The techniques of this disclosure allow a base station to initiate ahandover of a UE after determining that a UE has detected SCG failureand suspended a DRB due to the SCG failure. The base station indifferent scenarios initiates a handover from one cell of a base stationto another cell of the same base station, one DU of a disaggregated basestation to another DU of the same disaggregated base station, or onebase station to another base station. In the latter case, the basestation can initiate the handover using the core network (CN) or withoutusing the CN.

More particularly, a first base station communicating with a UE on aserving cell first can determine that a UE receives a signal of certainstrength and/or quality from a second base station and, in response,configure the UE with an SCG and a DRB for communicating with the MN(i.e., the first base station) and SN (i.e., the second base station) inDC. While operating in DC, the UE measures the signal strength and/orquality for the MCG and the SCG and, additionally, may measure thestrength and/or quality of at least one downlink carrier of other basestations. In an example scenario, the UE detects SCG failure andsuspends the DRB over which the UE communicates data with the SN. The UEnotifies the base station of the SCG failure by transmitting a SCGfailure indication to the first base station. In some cases, the UEprovides to the first base station at least one measurement result whichindicates the signal strength and/or quality for at least one downlinkcarrier or at least one cell. The UE can provide the first base stationthe at least one measurement result along with the SCG failureindication, or in one or more measurement report prior to the SCGfailure indication or after the SCG failure indication, for example.

When the first base station determines that the UE receives bettersignal quality and/or signal strength at a downlink carrier of at leastone other carrier or cell according to one or more measurement results,the base station initiates a handover procedure for the UE even thoughthe DRB is suspended as indicated in the SCG failure indication.Depending on the scenario, the base station can hand over the UE to thefirst cell or a second cell operated on the downlink carrier. Theserving cell, the first cell or the second cell may belong to the sameDU or different DUs, and can belong to the first base station or todifferent base stations (e.g., the first base station and a third basestation) that use the same radio access technology (RAT) or differentRATs. In one example, the UE and the first base station use a first RATto communicate with each other before the handover. The downlink carriermay be the first RAT downlink carrier or a second RAT downlink carrier.The first RAT can be fifth generation (5G) new radio (NR) and the secondRAT can be Evolved Universal Terrestrial Radio Access (E-UTRA), forexample.

In some cases, the base station additionally checks certain conditionsto determine whether the base station should allow the handoverprocedure when the DRB is suspended. In one such implementation, thebase station checks whether the UE reports the SCG failure in adedicated RRC message or reports an MCG failure in a message defined forreestablishing a radio connection. In another implementation, the basestation checks the reason for DRB suspension and allows the handover ifthe UE suspended the DRBs due to the SCG failure. In yet anotherimplementation, the base station checks whether the UE suspended all ofthe DRBs, and whether any of the suspended DRBs are split DRBs or MCGDRBs.

One example embodiment of these techniques is a method for performing ahandover procedure, which can be executed by processing hardware in abase station that operates as an MN. The method includes configuring aUE that operates in DC with the MN and an SN with a DRB associated withan SCG of the SN. The method further includes determining that the DRBhas been suspended, determining that a handover procedure should beperformed for the UE, and initiating the handover procedure when the DRBis suspended.

Another example embodiment of these techniques is a base station withhardware configured to execute the method above.

Yet another example embodiment of these techniques is a method forperforming a handover procedure. The method can be implemented in a UEoperating in DC with an MN and an SN. The method can be executed byprocessing hardware and includes steps such as obtaining a DRBassociated with an SCG of the SN; determining a failure of the SCG; inresponse to the failure, suspending the DRB; transmitting an indicationof the failure to the MN; and performing a handover procedure when theDRB is suspended.

Still another example embodiment of these techniques is a UE withhardware configured to execute the method above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of an example wireless communication networkin which a UE operates in dual connectivity with a master node and asecondary node, and the base station supports a handover procedure forthe UE when the cell group of the secondary node fails, in one exampleconfiguration;

FIG. 1B is a block diagram of the base station of FIG. 1A in anotherexample configuration;

FIG. 1C is a block of the base station of FIG. 1 in another exampleconfiguration;

FIG. 1D is a block diagram of the UE of FIG. 1A in an exampleconfiguration;

FIG. 2 is a messaging diagram of an example scenario in which a basestation implemented as a master node determines that the UE hassuspended a DRB and initiates a handover to another base station;

FIG. 3 is a messaging diagram of an example scenario in which a basestation implemented as a master node determines that the UE hassuspended a DRB and initiates a handover from one cell to another cell;

FIG. 4 is a messaging diagram of an example scenario in which a basestation implemented according to disaggregated architecture determinesthat the UE has suspended a DRB and initiates a handover from onedistributed unit to another distributed unit;

FIG. 5 is a messaging diagram of an example scenario in which a basestation implemented as a master node determines that the UE hassuspended a DRB and initiates a handover to another base station usingthe core network;

FIG. 6 is a messaging diagram of an example scenario in which a basestation implemented as a master node determines that the UE hassuspended a DRB and initiates a handover to another base station thatoperates according to a different RAT;

FIG. 7 is a flow diagram of an example method in a base station fordetermining whether the base station should allow a handover based onthe type of the message that reports secondary cell group failure;

FIG. 8 is a flow diagram of an example method in a base station fordetermining whether the base station should allow a handover based onwhether all of the DRBs of the UE are in a suspended state;

FIG. 9 is a flow diagram of an example method in a base station fordetermining whether the base station should allow a handover based onthe reason why DRBs are in a suspended state;

FIG. 10 is a flow diagram of an example method in a base station forinitiating a handover of a UE following a secondary cell group failure;and

FIG. 11 is a flow diagram of an example method in a UE for performing ahandover procedure following a secondary cell group failure.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts an example wireless communication network 100 in whichan example UE 102 operates in dual connectivity (DC) with a base stationoperating as a master node (MN) 104 and another base station operatingas a secondary node (SN) 106. The UE 102 implements the techniques ofthis disclosure to report SCG failures and measurement reports to the MNeven when the SCG has been suspended, and the base station 104implements the techniques of this disclosure to support a handoverprocedure when the SCG of the SN 106 has failed.

These and other related techniques are discussed below primarily withreference to a 5G communication that includes a 5G core network (5GC)and base stations that support a 5G NR radio access technology (RAT).However, at least some of these techniques also apply to other types ofbase stations, RATs, and core networks. Further, in the followingdescription, the terms “carrier,” “carrier frequency,” and “cell” can beused interchangeably.

In an example implementation, the base stations 104 and 106 operate as5G nodes B (gNBs). In a DC configuration, the gNBs 104 and 106 canoperate as an MN and an SN, respectively, and thus the gNB 104 operatesas a MgNB and the gNB 106 operates as an SgNB. The wirelesscommunication network 100 in general can include any suitable number ofbase stations, including a base station 108 which is for example a gNB,or a next-generation evolved Node B (ng-eNB). In the discussion thatfollows, base station 108 may be referred to as “gNB 108” or as“gNB/ng-eNB 108” to indicate that, in the implementation underdiscussion, base station 108 is a gNB, or base station 108 canalternatively be a gNB or an ng-eNB, respectively. Each of the basestations 104, 106, and 108 is connected to a 5GC 110. The gNBs and theng-eNBs in the wireless communication network 100 can use an NGinterface to exchange messages with the 5GC 110, and an Xn interface todirectly (i.e., without using the 5GC 110) exchange messages with othergNBs or ng-eNBs.

The wireless communication network 100 in some cases can include anevolved packet core (EPC) 120 and one or more eNBs including an eNB 122.The eNBs in the wireless communication network 100 can use an S1interface to exchange messages with the EPC 120, and an X2 interface todirectly (i.e., without using the EPC 120) exchange messages with othereNBs.

The MgNB 104 covers an MCG 124 made up of one or more cells, and theSgNB 106 covers an SCG 126 made up of one more cells. In this exampleconfiguration, the MCG 124 includes at least cells 124A and 124B. Thebase station 108 can cover a cell 128 and, in some cases, one or moreadditional cells. The MCG 124, the SCG 126, and the cell 128 can have acertain geographic overlap so that the UE 102 at some point can generatemeasurement reports for at least one carrier for each of the MCG 124,the SCG 126, and the cell 128. The eNB 122 can cover a cell 136 and, insome cases, one or more additional cells. The MCG 124, the SCG 126, andthe cell 136 can have a certain geographic overlap so that the UE 102 atsome point can generate measurement reports for at least one carrier foreach of the MCG 124, the SCG 126, and the cell 136.

The MgNB 104 is equipped with processing hardware that can implement anHO controller 132 and an RRC entity 134. In operation, the HO controller132 can determine whether a handover procedure should proceed after theUE 102 has reported SCG failure. The RRC entity 134 can generatecorresponding downlink RRC messages for the UE 102 (and/or other devicesoperating in the MCG 124) and process uplink RRC messages from the UE102.

In other implementations, one or more of the base stations 104, 106, and108 can be implemented as an eNB or an ng-eNB to support a EUTRA radiointerface in the corresponding cells. The base station 104 accordinglycan operate as a master eNB, a master ng-eNB or a master gNB, and/or thebase station 106 can operate as a secondary eNB, a secondary ng-eNB or asecondary gNB.

FIG. 1B illustrates an example implementation of processing hardware 140which the MgNB 104 can include. In this configuration, an HO controller142 interacts with multiple radio resource control RRC entities (ormodules) 143, 144, each responsible for a respective cell. The HOcontroller 142 can be implemented similar to the HO controller 132depicted in FIG. 1A, and the RRC entities 143, 144 can be similar to theRRC entity 134 of FIG. 1A. In other implementations, however, a singleRRC entity supports multiple cells of the base station.

FIG. 1C an example disaggregated configuration 150 of the gNB 106.According to this configuration, a gNB central unit (gNB-CU) 152implements the HO controller 132 and connects to gNB distributed units(gNB-DUs) 154 and 156 via F1 interface links. In general, the gNB-CU 152can support one, two, or more gNB-DUs. The gNB-DUs 154 and 156 in thisimplementation are sufficiently separated geographically to coverdifferent cells, which may have a certain geographic overlap.

The gNB-CU 152 in this architecture is a logical node hosting RRC,Service Data Adaptation Protocol (SDAP) and Packet Data ConvergenceProtocol (PDCP) protocols of the gNB; and the gNB-DU 154 or 156 is alogical node hosting radio link control (RLC), medium access control(MAC) and physical (PHY) layers of the gNB 104. The gNB-CU 152 partlycontrols operation of the gNB-DUs 154 and 156. The UE 102 communicateswith the gNB-DU 154 or 156 via a radio interface that conforms to the NRstandard, and the gNB-DU 154/156 exchanges messages with thecorresponding gNB 152.

As illustrated in FIG. 1D, the UE 102 in some implementations isequipped with processing hardware 160 which includes an SCG failurecontroller 162 and a UE HO controller 164. The SCG failure controller162 can detect SCG failures, control DRB suspension, etc. The UE HOcontroller 164 can exchange messages with the radio access network (RAN)in which the gNBs 104, 106, and 108 operate and/or the 5GC 110 regardinghandover procedures.

In the examples above, the processing hardware 130, 140, or 160 caninclude one or more general-purpose processors such as centralprocessing units (CPUs) and non-transitory computer-readable memorystoring machine-readable instructions executable on the one or moregeneral-purpose processors. Additionally or alternatively, theprocessing hardware 130, 140, or 160 can include special-purposeprocessing units implemented in hardware, firmware, software, or anysuitable combination of hardware, firmware, or software.

Several example scenarios according to which the UE 102 and the MgNB 106can carry out procedures related to handover during an SCG failure arediscussed next with reference to FIGS. 2-5.

In a scenario 200 of FIG. 2, the gNB 104 can configure the UE 102 togenerates at least one measurement result for a downlink (DL) carrierfrequency of the gNB 106 (not shown in FIG. 2 to avoid clutter). The UE102 generates the at least one measurement result and transmits 202 aMeasurement Report message to the gNB 104, in accordance with thisconfiguration. The Measurement Report message can indicate signalstrength and/or signal quality on the DL carrier frequency of the gNB106. The UE 102 also can send one or more instances of the MeasurementReport message to the gNB 104.

Based on the received measurement report, the gNB 104 can determine 204that the UE 102 should operate in DC, with the gNB 104 operating as anMgNB and the gNB 106 operating as an SN. The gNB 104 initiates an SNaddition procedure with the gNB 106 to establish a context for the UE102 at the gNB 106 and provide radio resources of the gNB 106 to the UE102. In particular, the gNB 104 sends 206 an SN Addition Request messageto the gNB 106. In response, the gNB 106 sends 208 an SN AdditionRequest Ack message to the gNB 104, and the gNBs 104 and 106 at thispoint are prepared operate as an MgNB and an SgNB, respectively. In someimplementations, the SN Addition Request message is an S-Node AdditionRequest message, and the SN Addition Request Ack message is a S-NodeAddition Request Acknowledge message.

The MgNB 104 configures 212 the UE 102 with a DRB for communicating withthe SgNB 106 and provides a configuration for the SCG associated withthe SgNB 106. To this end, the MgNB 104 sends to the UE 102 an RRCReconfiguration message including a cell group configuration(CellGroupConfig) information element (IE) for the SCG and aconfiguration of a DRB (e.g., a Radio Bearer Configuration(RadioBearerConfig) IE or a DRB-ToAddMod IE) for the SCG (i.e., the DRBcan be a split DRB or a SCG DRB). The UE 102 can transmit 214 an RRCReconfiguration Complete message in response. In some implementations,the MgNB 104 receives the CellGroupConfig IE and the configuration ofthe DRB in the SN Addition Ack message if the DRB is a split DRB or aSCG DRB. That is, the SgNB 106 configures the DRB to the UE via the MgNB104.

The UE 102 and the SgNB 106 then perform 220 a random access procedure,which can be any suitable random procedure that includes an exchange oftwo message, four messages, etc. Upon successfully completing the randomaccess procedure, the UE 102 begins to operate 230 in DC with the MgNB104 and the SgNB 106. Specifically regarding the SCG, the UE 102 and theSgNB 106 begin to exchange 232 PDCP PDUs over the DRB of the SCG. Theevents 202-232 collectively make up a DC setup procedure 240. In someimplementations, the UE 102 performs the random access procedureaccording to the CellGroupConfig IE if the CellGroupConfig IE configuresrandom access resources for the UE 102 to perform 220 the random accessprocedure. In other implementations, the UE 102 may skip the randomaccess procedure if the CellGroupConfig IE configures the UE to do so.The UE skips the random access procedure accordingly.

At a later time and while operating in DC with the MgNB 104 and the SgNB106, the UE 102 detects 250 SCG failure on a radio connection betweenthe UE 102 and the SgNB 106. For example, the UE 102 can detect an SCGradio link failure (RLF), reconfiguration with sync failure, integritycheck failure, or SCG configuration failure, on the radio connection tothe SgNB 106. In response, the UE 102 suspends 252 the DRB.

The UE 102 also provides 254 an indication to the MgNB 104 that the SCGhas failed. The UE 102 in some implementations sends an SCG FailureInformation message to the MgNB 104. In some cases, the SCG FailureInformation message (or another indication of the SCG failure) caninclude at least one measurement result (or report) indicating that theUE 102 measures a relatively low strength and/or quality on DL carrierfrequency of the MgNB 104, and a relatively high strength and/or qualityon a DL carrier frequency of the gNB/ng-eNB 108. The DL carrierfrequency of the MgNB 104 and the DL carrier frequency of the gNB/ng-eNB108 can be same or different. In some implementations, the at least onemeasurement result may include a physical cell identity (PCI), asynchronization signal block (SSB) index, a channel state informationreference signal (CSI-RS) index, reference signal received power (RSRP),reference signal received quality (RSRQ), signal to interference andnoise ratio (SINR) and/or an event identity (e.g., so-called events A3,A4 or A5) when the base station 108 is a gNB. When the base station 108is an ng-eNB, the at least one measurement result may include a PCI,RSRP, RSRQ, and/or an event identity (e.g., so-called events B1 or B2).

After receiving an indication that the SCG has failed, the MgNB 104determines 258 that the UE 102 has suspended the DRB.

As an alternative to including at least one measurement result fordownlinks signals from the MgNB 104 and the gNB/ng-eNB 108 in an SCGFailure Information message, or in addition to doing so, the UE 102 cansend 260 to the MgNB 104 a Measurement Report message including the atleast one measurement result or similar measurement results, afterreporting the SCG failure to the MgNB 104. The UE 102 also can send oneor more instances of the Measurement Report message to the MgNB 104prior to detecting the SCG failure. In some implementations, the UE 102performs the measurements to get the at least measurement result inaccordance with the configuration the MgNB 104 has provided.

The MgNB 104 determines 264 that the UE 102 measures better signalquality and/or strength on a downlink carrier of the gNB/ng-eNB 108 thanin the cell of the MCG 124. The MgNB 104 then determines 266 that ahandover to the gNB/ng-eNB 108 is desirable.

The MgNB 104 sends 270 a Handover Request message to the gNB/ng-eNB 108.The gNB/ng-eNB 108 can generate an RRC Reconfiguration message includinga reconfigurationWithSync IE and encapsulate the RRC Reconfigurationmessage in a Handover Request Acknowledge message. The RRCReconfiguration message indicates a cell in which the UE 102 shouldperform a random access procedure. The gNB/ng-eNB 108 then responds 272with the Handover Request Acknowledge. The MgNB 104 then can forward 290the RRC Reconfiguration message from the gNB/ng-eNB 108 to the UE 102,thereby instructing the UE 102 to hand over to the gNB/ng-eNB 108.

The UE 102 performs 294 a random access procedure in the cell specifiedin the RRC Reconfiguration message and, after completing this proceduresuccessfully, the UE 102 sends 298 an RRC Reconfiguration Completemessage to the gNB/ng-eNB 108. Thus, the MgNB 104 in this scenariocoordinates a procedure to hand the UE 102 over to gNB/ng-eNB 108 eventhough the DRB associated with the SCG has been suspended. In someimplementations, the gNB/ng-eNB 108 may configure random accessresources in the RRC Reconfiguration message so that the UE 102 performs294 the random access procedure according to the random accessresources. If the gNB/ng-eNB 108 indicates that the UE 102 should skipthe random access procedure in the RRC Reconfiguration message, the UE102 does not perform 294 the random access procedure. For example, thegNB/ng-eNB 108 may not configure the random access resources in the RRCReconfiguration message to indicate that the UE 102 should not performthe random access procedure.

The RRC Reconfiguration message may configure the DRB to be an MCGbearer. In some implementations, the UE 102 may resume the suspended DRBin response to the RRC Reconfiguration message. In otherimplementations, the UE may resume the suspended DRB in response to therandom access procedure or after the random access procedure. When theDRB is resumed (i.e., not suspended), the UE 102 transmits to thegNB/ng-eNB 108 data associated with the suspended DRB after the RRCReconfiguration Complete message or along with the RRC ReconfigurationComplete message.

Next, FIG. 3 illustrates an example scenario 300 that is generallysimilar to the scenario 200 discussed above, except that here an MN,after determining that the UE has suspended a DRB which can be split DRBor a SCG DRB, initiates a handover from one cell to another cell. Forclarity, FIG. 3 refers to two separate RRC entities or controllers inconnection with the two cells; however, the gNB 104 in otherimplementations can rely on a single RRC entity for both cells.

The UE 102, the RRC entity 142 of the MgNB 104 (or simply, “MgNB cell1”), and the SgNB 106 can set up 340 dual connectivity for the UE 102,similar to the procedure 240 of FIG. 2. When operating in DC, the UE 102detects 350 SCG failure, suspends 352 the DRB, and provides 354 to theMgNB cell 1 an indication that the SCG has failed. The MgNB cell 1determines 358 that the SCG has failed and the UE 102 suspended the DRB,based on the received indication. The events 350, 352, 354, and 358 aresimilar to the events 250, 252, 254, and 258, respectively.

The UE 102 sends 360 to the MgNB cell 1 a Measurement Report messageprior to sending 354 an indication of SCG failure or after sending 354the indication of SCG failure. The Measurement Report message and/or theSCG Failure Information message includes at least one downlink carriermeasurement result. In this scenario, the at least one measurementresult in the Measurement Report message or in the SCG FailureInformation message can include one or more downlink carrier measurementresults for another cell of the MgNB 104. For example, if the UE 102operates in the cell 124A when setting up 340 dual connectivity with theMgNB cell 1 and the SgNB 106, the SCG Failure Information message or theMeasurement Report can include at least one downlink carrier measurementresult for the cell 124B. Depending on the configuration or scenario,downlink carrier frequencies in the cells 124A and 124B can be the sameor different.

The MgNB cell 1 determines 364 that the UE 102 measures better signalquality and/or strength on a downlink carrier in another cell (e.g., thecell 124B) than in the current cell of the MgNB 104 in which the UE 102operates (e.g., the cell 124A). The MgNB cell 1 then determines 366 thata handover to the new cell is desirable.

The MgNB cell 1 sends 390 an RRC Reconfiguration message to the UE 102.This message can include a reconfigurationWithSync IE and an indicationof the target cell (i.e., the MgNB cell 2) to which the UE 102 shouldhand over. The UE 102 then performs 394 a random access procedure withthe RRC entity 144 of the gNB 104 (or simply, “MgNB cell 2”). During therandom access procedure, the UE 102 can send a random access preamble tothe MgNB 104 via the cell 124B. After completing this proceduresuccessfully, the UE 102 sends 398 an RRC Reconfiguration Completemessage to the MgNB cell 2. In this manner, the MgNB 104 coordinates ahandover of the UE 102 to a new cell even though the DRB associated withthe SCG has been suspended. In some implementations, the MgNB 104 mayconfigure random access resources in the RRC Reconfiguration message sothat the UE 102 performs 394 the random access procedure according tothe random access resources. If the MgNB 104 indicates the UE 102 toskip the random access procedure in the RRC Reconfiguration message, theUE 102 does not perform 394 the random access procedure. For example,the gNB 108 may not configure the random access resources in the RRCReconfiguration message to indicate that the UE 102 should not performthe random access procedure.

The RRC Reconfiguration message may configure the DRB to be an MCGbearer. In some implementations, the UE 102 may resume the suspended DRBin response to the RRC Reconfiguration message. In otherimplementations, the UE may resume the suspended DRB in response to therandom access procedure or after the random access procedure. When theDRB is resumed (i.e., not suspended), the UE 102 transmits the MgNB 104data associated the suspended DRB after the RRC Reconfiguration Completemessage or along with the RRC Reconfiguration Complete message.

Next, FIG. 4 illustrates of an example scenario 400 in which the MgNB104, implemented in a disaggregated mode as illustrated in FIG. 1C forexample, initiates a handover from one distributed unit 154 of the MgNB104 to another distributed unit 156 of the MgNB 104, after determiningthat the UE 102 has suspended a DRB.

In this scenario, the UE 102, the MgNB 104, and the SgNB 106 first setup 440 dual connectivity for the UE 102. This procedure is similar tothe procedure 240 of FIG. 2, but because the MgNB 104 in this scenariois disaggregated, the UE 102 sends a Measurement Report message to theMgNB-CU 152 via the MgNB-DU 154. Also, in the downlink direction, theMgNB-CU 152 sends an RRC Reconfiguration message with the cell group andDRB configuration to the UE 102 via the MgNB-DU 154.

Similar to the events 250 and 252, the UE 102 detects 450 SCG failureand suspends 452 a DRB which can be a split DRB or a SCG DRB. The UE 102then sends 454 an SCG Failure Information message to the MgNB-DU 154,which in turn sends 456 the SCG Failure Information message to theMgNB-CU 152. The SCG Failure Information message (or, in otherimplementations, another indication of the SCG failure the UE 102 sendsto the MgNB-CU 154) can include at least one measurement resultindicating that the UE 102 measures a relatively low strength and/orquality on DL carrier frequency of the MgNB-DU 154, and a relativelyhigh strength and/or quality on a DL carrier frequency of the MgNB-DU156. Depending on the configuration or scenario, the DL carrierfrequencies in MgNB-DU 154 and MgNB-DU 156 can be the same or different.After receiving an indication that the SCG has failed, the MgNB-CU 152determines 458 that the UE 102 has suspended the DRB.

As an alternative to including measurement results for downlinks signalsfrom the MgNB-DU 154 and the MgNB-DU 156 in an SCG Failure Informationmessage, or in addition the doing so, the UE 102 can send 460 to theMgNB-DU 154 a Measurement Report message including the at least onemeasurement result or similar measurement results to the MgNB-DU 154.The UE 102 sends one or more instances of the Measurement Report messageto the MgNB-DU 154 prior to detecting the SCG failure, after detectingthe SCG failure but prior to reporting the SCG failure, or afterdetecting and reporting the SCG failure. In some implementations, the UE102 performs the measurements to get the at least measurement result inaccordance with the configuration the MgNB-CU 152 has provided.

In any case, the MgNB-CU 152 determines 464 that the UE 102 measuresbetter signal quality and/or strength on a downlink carrier of theMgNB-DU 156 than on downlink carrier of the MgNB-DU 154. In view of thisdetermination, the MgNB-CU 152 then determines 466 that a handover tothe MgNB-DU 156 is desirable.

The MgNB-CU 152 sends 480 a UE Context Setup Request message to theMgNB-DU 156. In response, the MgNB-DU 156 generates a CellGroupConfig IEincluding a reconfigurationWithSync IE. The CellGroupConfig IE indicatesa cell in which the UE 102 should perform a random access procedure. TheMgNB-DU 156 includes the CellGroupConfig IE in a UE Context SetupResponse message and sends 482 the UE Context Setup Response message tothe MgNB-CU 152.

The MgNB-CU 152 sends 492 to the UE 102 an RRC Reconfiguration messageincluding the CellGroupConfig IE, via the MgNB-DU 154 (event 490). TheMgNB-CU 152 thereby instructs the UE 102 to perform a handover to theMgNB-DU 156. More specifically, after receiving the RRC Reconfigurationmessage, the UE 102 performs 494 a random access procedure in the cellof the MgNB-DU 156 specified in the RRC Reconfiguration message and,after completing this procedure successfully, the UE 102 sends 496 anRRC Reconfiguration Complete message to the MgNB-DU 156 which theMgNB-DU 156 forwards 498 the RRC Reconfiguration Complete message to theMgNB-CU 152. Thus, the MgNB-CU 152 in this scenario coordinates aprocedure to hand the UE 102 over from the MgNB-DU 154 to the MgNB-DU156 even though the DRB associated with the SCG has been suspended. Insome implementations, the MgNB-DU 156 may configure random accessresources in the CellGroupConfig IE so that the UE 102 performs 294 therandom access procedure according to the random access resources. If theMgNB-DU 156 indicates the UE 102 should not perform the random accessprocedure in the CellGroupConfig IE, the UE 102 does not perform 294 therandom access procedure. For example, the MgNB-DU 156 may not configurethe random access resources in the CellGroupConfig IE to indicate thatthe UE 102 should not performthe random access procedure.

The RRC Reconfiguration message may configure the DRB to be an MCGbearer. In some implementations, the UE 102 may resume the suspended DRBin response to the RRC Reconfiguration message. In otherimplementations, the UE may resume the suspended DRB in response to therandom access procedure or after the random access procedure. When theDRB is resumed (i.e., not suspended), the UE 102 transmits the MgNB-DU156 data associated the suspended DRB after the RRC ReconfigurationComplete message or along with the RRC Reconfiguration Complete message.

In some implementations, the MgNB-CU 152 also sends a UE Context ReleaseCommand message to the MgNB-DU 154 after sending the RRC Reconfigurationmessage to the UE 102 or after receiving the RRC ReconfigurationComplete message (not shown in FIG. 4 to avoid clutter). The MgNB-DU 154releases the context of the UE 102 and/or the configuration of the UE102 and sends a UE Context Release Complete message to the MgNB-DU 152,in response to the UE Context Release Command message.

FIG. 5 illustrates an example scenario 500 that is generally similar tothe scenario 200, in which the MgNB 104 determines that the UE 102should perform a handover procedure to the gNB/ng-eNB 108, but here theMgNB 104 uses the 5GC 110.

The UE 102, the MgNB 104, and the SgNB 106 can set up 540 dualconnectivity for the UE 102, similar to the procedure 240 of FIG. 2.When operating in DC, the UE 102 detects 550 SCG failure, suspends 552the DRB which can be a split DRB or a SCG DRB, and provides 554 to theMgNB 104 an indication that the SCG has failed. The MgNB 104 determines558 that the SCG has failed, based on the received indication. The UE102 also can transmit 560 to the MgNB 104 one or more MeasurementReports, indicating the strength and/or quality of downlink carriersignals for the MgNB 104 and/or gNB/ng-eNB 108. The MgNB 104 determines564 that the UE 102 measures better signal quality and/or strength on adownlink carrier frequency (or a cell) of the gNB 108 than in the cellof the MCG 124. The MgNB 104 then determines 566 that a handover to thegNB/ng-eNB 108 is desirable. The events 550, 552, 554, 558, 560, 564,and 566 are similar to the events 250, 252, 254, 258, 260, 264, and 266,respectively.

The MgNB 104 then sends 574 a Handover Required message to the 5GC 110.More particularly, the MgNB 104 can send the Handover Required messageto the Access and Mobility Management Function (AMF) operating in the5GC 110. In response, the 5GC 110 sends 576 a Handover Request messageto the target gNB/ng-eNB 108.

The gNB/ng-eNB 108 generates a ReconfigurationWithSync IE, includes theReconfigurationWithSync IE in an RRC Reconfiguration message, includesthe RRC Reconfiguration message in a Handover Request Acknowledgemessage, and sends 578 the Handover Request Acknowledge message to the5GC 110. The RRC Reconfiguration message includes an indication of acell of the gNB/ng-eNB 108 on which the UE 102 should perform a randomaccess procedure. The 5GC110 extracts the RRC Reconfiguration messagefrom the Handover Request Acknowledge message and includes the RRCReconfiguration message in a Handover Command message. The 5GC 110 thensends 579 the Handover Command message to the MgNB 104. In response, theMgNB 104 sends 590 an RRC Reconfiguration message to the UE 102, therebyinstructing the UE 102 to hand over to the gNB/ng-eNB 108.

The UE 102 performs 594 a random access procedure in the cell specifiedin the RRC Reconfiguration message and, after completing this proceduresuccessfully, the UE 102 sends 598 an RRC Reconfiguration Completemessage to the gNB 108. Thus, the MgNB 104 in this scenario coordinatesa procedure to hand the UE 102 over to gNB/ng-eNB 108 even though theDRB associated with the SCG has been suspended. In some implementations,the gNB/ng-eNB 108 may configure random access resources in the RRCReconfiguration message so that the UE 102 performs 594 the randomaccess procedure according to the random access resources. If thegNB/ng-eNB 108 indicates that the UE 102 should not perform the randomaccess procedure in the RRC Reconfiguration message, the UE 102 does notperform 594 the random access procedure. For example, the gNB/ng-eNB 108may not configure the random access resources in the RRC Reconfigurationmessage to indicate that the UE 102 should not perform the random accessprocedure.

The RRC Reconfiguration message may configure the DRB to be an MCGbearer. In some implementations, the UE 102 may resume the suspended DRBin response to the RRC Reconfiguration message. In otherimplementations, the UE may resume the suspended DRB in response to therandom access procedure or after the random access procedure. When theDRB is resumed (i.e., not suspended), the UE 102 transmits thegNB/ng-eNB 108 data associated the suspended DRB after the RRCReconfiguration Complete message or along with the RRC ReconfigurationComplete message.

FIG. 6 illustrates an example scenario 600 that is generally similar tothe scenario 200, in which the MgNB 104 determines that the UE 102should perform a handover procedure to the eNB 122.

The UE 102, the MgNB 104, and the SgNB 106 can set up 640 dualconnectivity for the UE 102, similar to the procedure 240 of FIG. 2.When operating in DC, the UE 102 detects 650 SCG failure, suspends 652the DRB which can be a split DRB or a SCG DRB, and provides 654 to theMgNB 104 an indication that the SCG has failed. The MgNB 104 determines658 that the SCG has failed, based on the received indication. The UE102 also can transmit 660 to the MgNB 104 one or more MeasurementReports, indicating the strength and/or quality of downlink carriersignals for the MgNB 104 and/or the eNB 122. The MgNB 104 determines 664that the UE 102 measures better signal quality and/or strength on adownlink carrier frequency (or a cell) of the eNB 122 than in the cellof the MCG 124. The MgNB 104 then determines 666 that a handover to theeNB 122 is desirable. The events 650, 652, 654, 658, 660, 664, and 666are similar to the events 250, 252, 254, 258, 260, 264, and 266,respectively.

The MgNB 104 then sends 674 a Handover Required message to the 5GC 110.More particularly, the MgNB 104 can send the Handover Required messageto the AMF operating in the 5GC 110. In response, the 5GC 110 sends 675a Forward Relocation Request message to the EPC 120. The EPC 120 thensends 676 a Handover Request message to the target eNB 122.

The eNB 122 generates a MobilityControlInfo IE, includes theMobilityControlInfo IE in an RRC Connection Reconfiguration message,includes the RRC Connection Reconfiguration message in a HandoverRequest Acknowledge message, and sends 677 the Handover RequestAcknowledge message to the EPC 120. The RRC Connection Reconfigurationmessage includes an indication of a cell of the eNB 122 on which the UE102 should perform a random access procedure. The EPC 120 extracts theRRC Connection Reconfiguration message from the Handover RequestAcknowledge message and includes the RRC Connection Reconfigurationmessage in a Forward Relocation Response message. The EPC 120 sends 678the Forward Relocation Response message to the 5GC 110. The 5GC 110extracts the RRC Connection Reconfiguration message from the ForwardRelocation Response message and includes the RRC ConnectionReconfiguration message in a Handover Command message. The 5GC 110 thensends 679 the Handover Command message to the MgNB 104. In response, theMgNB 104 sends 690 the RRC Connection Reconfiguration message to the UE102, thereby instructing the UE 102 to hand over to the eNB 122. In someimplementations of the event 690, the MgNB 104 includes the RRCConnection Reconfiguration message in a Mobility From NR Command messageand sends the Mobility From NR Command message to the UE 102.

The UE 102 performs 694 a random access procedure in the cell specifiedin the RRC Connection Reconfiguration message and, after completing thisprocedure successfully, the UE 102 sends 698 an RRC ConnectionReconfiguration Complete message to the eNB 122. Thus, the MgNB 104 inthis scenario coordinates a procedure to hand the UE 102 over to the eNB122 even though the DRB associated with the SCG has been suspended. Insome implementations, the eNB 122 may configure random access resourcesin the RRC Connection Reconfiguration message, so that the UE 102performs 694 the random access procedure according to the random accessresources. If the eNB 122 indicates that the UE 102 should not performthe random access procedure in the RRC Connection Reconfigurationmessage, the UE 102 does not perform 694 the random access procedure.For example, the eNB 122 may not configure the random access resourcesin the RRC Connection Reconfiguration message to indicate that the UE102 should not perform the random access procedure.

The RRC Connection Reconfiguration message the eNB 122 generates mayconfigure a new DRB to replace the suspended DRB. The UE 102 replacesthe suspended DRB with the new DRB in response to the RRC ConnectionReconfiguration message. For example, the UE 102 can exchange data of anapplication (e.g., a web browser, a video streaming application, amessaging application, a social media application) over a certain DRBwith the SgNB 106, before the handover. The UE 102 suspends the DRB inresponse to the SCG failure. Consequently, the UE 102 suspendsexchanging data of the application. After the handover, the UE 102exchanges data of the application over the new DRB with the eNB 122. Inone implementation, the UE 102 uses the new DRB to exchange data withthe eNB 122 in response to the RRC Connection Reconfiguration message.In other implementations, the UE 102 uses the new DRB to exchange datawith the eNB 122 in response to the random access procedure, or afterthe random access procedure. When the UE 102 uses the new DRB in theimplementations above, the UE 102 transmits to the eNB 122 data over thenew DRB after the RRC Connection Reconfiguration Complete message, oralong with the RRC Connection Reconfiguration Complete message. The UE102 also releases the suspended DRB, i.e., the UE 102 no longer uses theDRB in response to the RRC Connection Reconfiguration message. The UE102 resumes exchanging data of the application after the handover.

In some cases, the MgNB 104 may determine to initiate handover for theUE 102 because the 5GC 110 sent an appropriate request to an MgNB 104.For example, the 5GC 110 may request that the MgNB 104 hand over the UE102 to E-UTRA for an Internet Protocol (IP) Multimedia Subsystem (IMS)service (e.g., voice call or video call). Then, the MgNB 104 determinesto handover the UE 102 to the eNB 122 as described above.

Next, FIGS. 7-9 illustrate several example methods that can beimplemented in a base station to support the scenarios above, inaccordance with certain conditions. The discussion below references thebase station 104 which, as discussed above, can be a gNB, an ng-eNB, oreNB, for example. More generally, these methods can be implemented inany suitable base station.

Referring first to FIG. 7, a method 700 for determining whether the basestation should allow a handover based on the type of a message the UEuses to report failure can be implemented in the base station 104 ofFIG. 1, for example. More particularly, the base station 104 canimplement the method 700 for a handover between base stations, betweencells of the same base station, or between distributed units coupled tothe same central unit.

At block 702, the base station 104 receives, from the UE 102, anindication of failure (i.e., MCG failure or SCG failure). In somescenarios, the UE 102 reports the SCG failure to the base station 104using the SCG Failure Information message (see for example the events254, 354, 454, and 554 above). In other scenarios, the UE 102 can reportthe MCG failure to the gNB 104 using an RRC Reestablishment Requestmessage. The UE 102 can use different mechanisms to transmit the SCGFailure Information and RRC Reestablishment Request messages. Inparticular, the UE 102 can transmit the SCG Failure Information messageover an SRB1 established between the UE 102 and the base station 104,and transmit the RRC Reestablishment Request message over an SRB0established between the UE 102 and the base station 104. Further, the UE102 can transmit the SCG Failure Information message without firstperform a random access procedure, but the UE 102 performs a randomaccess procedure in order to transmit the RRC Reestablishment Requestmessage.

Next, at block 704, the base station 104 determines whether the messagethat indicates failure is SCG Failure Information or RRC ReestablishmentRequest. If the message is SCG Failure Information, the flow proceeds toblock 706, and the base station 104 determines that a handover for theUE 102 is allowed. As discussed above, the base station 104 in variousscenarios sends a Handover Request message to the base station 108 (seeFIG. 2) or a Handover Required message to the 5GC 110 (see FIG. 5, FIG.6), sends an RRC Reconfiguration to the UE 102 (see FIG. 3), or sends aUE Context Setup Request from the CU to a DU (see FIG. 4).

Otherwise, if the message is RRC Reestablishment Request, the flowproceeds to block 708, and the base station 104 determines that thehandover for the UE 102 is not allowed at this time.

Optionally, the flow further proceeds from block 708 to block 710, wherebase station 104 transmits an RRC Reestablishment message to the UE 102.The base station 104 transmits an RRC Reconfiguration message to the UE102, at block 712. The base station 104 does not include areconfigurationWithSync IE in the RRC Reconfiguration message becausethe base station 104 determined at block 708 that the UE 102 is notallowed to hand over to another cell or another DU. The UE 102 resumesthe suspended DRB and SRB2 (and, some cases, other configured butsuspended DRBs). Then, at block 714, the base station 104 receives anRRC Reconfiguration Complete message from the UE 102. The base station104 accordingly becomes aware that the UE 102 has resumed the DRB(s) andthe SRB2. In some implementations, the base station 104 transmits RRCReconfiguration message to the UE 102 prior to receiving from the UE anRRC Reestablishment Complete message in response to the RRCReestablishment message. In another implementation, the base station 104does not transmit RRC Reconfiguration message to the UE 102 prior toreceiving the RRC Reestablishment Complete message, and transmits theRRC Reconfiguration message to the UE 102 only after receiving RRCReestablishment Complete.

FIG. 8 illustrates an example method 800 for determining whether a basestation should allow a handover based on whether all of the DRBs of a UEare in a suspended state, which also can be implemented in the basestation 104. The base station 104 can execute the method 800 forhandovers to another base station, another cell of the same basestation, or between DUs of the same disaggregated base station.

At block 802, the base station 104 receives an RRC message from the UEindicating SCG failure. The RRC message can be for example SCG FailureInformation (see for example the events 254, 354, 454, 554, and 654above).

At block 804, the base station 104 determines whether all of the DRBsconfigured for the UE are suspended. If the base station 104 determinesthat all of the DRBs of the UE are suspended, the flow proceeds to block806. Otherwise, in response to determining that not all of the DRBs ofthe UE are suspended, the flow proceeds to block 808, and the basestation 104 determines that the UE 102 is allowed to hand over toanother base station and/or cell, or another DU.

At block 806, the UE 806 further checks whether any of the suspendedDRBs corresponds to an MCG bearer or a split bearer (i.e., a bearerassociated with the MCG as well as the SCG). When the base stationdetermines that none of the suspended DRBs is an MCG bearer or a splitbearer, the flow proceeds to block 808. Otherwise, when the suspendedDRBs include an MCG bearer or a split bearer, the flow proceeds to block810, and the base station 104 determines that a handover for the UE 102is not allowed. In some implementations, the split bearer terminates atthe MCG (i.e., PDCP of the split bearer at a MN) instead of the SCG(i.e., PDCP of the split bearer at a SN).

Next, FIG. 9 illustrates an example method 900 for determining whetherthe base station should allow a handover based on the reason why DRBsare in a suspended state, which also can be implemented in the basestation 104.

At block 902, the base station 104 receives an RRC message from the UEindicating SCG failure. The RRC message can be for example SCG FailureInformation (see for example the events 254, 354, 454, 554, and 654above).

At block 904, the base station 104 determines whether all of the DRBsconfigured for the UE are suspended, similar to block 804 discussedabove. If the base station 104 determines that all of the DRBs of the UEare suspended, the flow proceeds to block 906. Otherwise, in response todetermining that not all of the DRBs of the UE are suspended, the flowproceeds to block 908, and the base station 104 determines that the UE102 is allowed to hand over to another base station and/or cell, oranother DU.

At block 906, the base station 104 determines whether the reason forsuspending the DRBs for the UE 102 is SCG failure. If all of the DRBsare suspended due to the SCG failure, the flow proceeds to block 908,and the base station 104 determines that the UE 102 is allowed to handover to another base station and/or cell, or another DU. Otherwise, theflow proceeds to block 910, where the base station 104 determines thatthe UE 102 is not allowed to hand over to another base station and/orcell, or another DU.

In some situations, the base station 104 can implement more than one ofthe methods 700, 800, and 900, or implement a combination of thesemethods. For example, the base station can allow a handover if all ofthe conditions of blocks 704, 804, and 904 are satisfied.

FIG. 10 is a flow diagram of an example method 1000 for initiating ahandover of a UE following an SCG failure, which can be implemented inthe base station 104.

At block 1002, the base station 104 configures the UE 102 with a DRBassociated with an SCG (e.g., event 212 of FIG. 2, procedure 340 of FIG.3, procedure 440 of FIG. 4, procedure 540 of FIG. 5, procedure 640 ofFIG. 6). At block 1004, the base station 104 determines that the DRB issuspended (e.g., event 258 of FIG. 2, event 358 of FIG. 3, event 458 ofFIG. 4, event 558 of FIG. 5, event 658 of FIG. 6). The base station 104can make this determination based on an indication of SCG failure (e.g.,event 254 of FIG. 2, event 354 of FIG. 3, event 454 of FIG. 4, event 554of FIG. 5, event 654 of FIG. 6).

At block 1006, the base station 104 determines that the UE 102 and/orone or more components of RAN or the GCC 110 should perform a handoverprocedure involving the UE 102 (e.g., event 266 of FIG. 2, event 356 ofFIG. 3, event 456 of FIG. 4, event 558 of FIG. 5, event 658 of FIG. 6).The base station 104 can make this determination based at least in parton the quality and/or strength of signals on the downlink carriers ofthe current cell of the UE and a different cell in which the UE 102 canperform measurements (e.g., event 264 of FIG. 2, event 364 of FIG. 3,event 464 of FIG. 4, event 564 of FIG. 5, event 664 of FIG. 6). The basestation 104 then initiates 1008 the handover procedure when the DRB issuspended (e.g., event 270 of FIG. 2, event 390 of FIG. 3, event 480 ofFIG. 4, event 574 of FIG. 5, event 674 of FIG. 6).

Next, FIG. 11 illustrates an example method 1100 for performing ahandover procedure following an SCG failure, which can be implemented inthe UE 102.

At block 1102, the UE 102 obtains a radio bearer associated with an SCG(e.g., event 212 of FIG. 2, procedure 340 of FIG. 3, procedure 440 ofFIG. 4, procedure 540 of FIG. 5). At a later time, the UE 102 determinesa failure of the SCG (block 1104) (e.g., event 250 of FIG. 2, event 350of FIG. 3, event 450 of FIG. 4, event 550 of FIG. 5, event 650 of FIG.6). The UE 102 then suspends the DRB, at block 1106 (e.g., event 252 ofFIG. 2, event 352 of FIG. 3, event 452 of FIG. 4, event 552 of FIG. 5,event 652 of FIG. 6).

At block 1108, the UE 102 transmits an indication of failure of the SCGto the MN (e.g., event 254 of FIG. 2, event 354 of FIG. 3, event 454 ofFIG. 4, event 554 of FIG. 5, event 654 of FIG. 6). At block 1110, the UE102 performs a handover procedure when the DRB is suspended. To thisend, the UE 102 receives an RRC Reconfiguration message (event 290 ofFIG. 2, event 390 of FIG. 3, event 490 of FIG. 4, or event 590 of FIG.5) or an RRC Connection Reconfiguration message (event 690 of FIG. 6).The RRC Reconfiguration message or the RRC Connection Reconfigurationmessage indicates a new cell of another base station (e.g., thegNB/ng-eNB 108 or the eNB 122), the same base station (e.g., the MgNBcell 2 of the RRC entity 144), or a different DU of the same basestation (e.g., the MgNB-DU 156).

When the UE 102 receives the RRC Reconfiguration message or the RRCConnection Reconfiguration message indicating that the UE 102 shouldhand over to the new cell, the UE 102 can check whether the UE 102previously suspended one or more DRBs in response to the failure of theMCG or the SCG. If the UE 102 suspended the one or more DRBs due to anMCG failure, the UE 102 in one implementation ignores the RRCReconfiguration message, the RRC Connection Reconfiguration message, orthe Mobility From NR Command message. and performs an RRCreestablishment procedure. Otherwise, the UE 102 performs handover tothe new cell according to the RRC Reconfiguration message or the RRCConnection Reconfiguration message.

The UE 102 then performs a random access procedure on this new cell(e.g., procedure 292 of FIG. 2, procedure 394 of FIG. 3, procedure 494of FIG. 4, procedure 594 of FIG. 5, procedure 694 of FIG. 6). Duringthis procedure, the UE 102 transmits a random access preamble on the newcell and receives a random access response on the new cell. The responseincludes an uplink grant. The UE 102 then transmits an RRCReconfiguration Complete message to the base station on the new cell(e.g., event 298 of FIG. 2, event 398 of FIG. 3, events 496, 498 of FIG.4, event 598 of FIG. 5) or an RRC Connection Reconfiguration message(e.g., event 698 of FIG. 6). Thus, rather than determining that the RRCReconfiguration message is invalid due to the suspension of the DRB, theUE 102 according to the method 1100 hands over to the new cell.

Further, as discussed above, the UE 102 in some cases performs an RRCreestablishment procedure by sending an RRC Reestablishment Requestmessage. The UE 102 then receives an RRC Reestablishment message inresponse and sends an RRC Reestablishment Complete message in response.As another example, the UE 102 can receive an RRC Setup message inresponse to the RRC Reestablishment Request message and, in response,sends an RRC Setup Compete message.

The following additional considerations apply to the foregoingdiscussion.

A user device in which the techniques of this disclosure can beimplemented (e.g., the UE 102) can be any suitable device capable ofwireless communications such as a smartphone, a tablet computer, alaptop computer, a mobile gaming console, a point-of-sale (POS)terminal, a health monitoring device, a drone, a camera, amedia-streaming dongle or another personal media device, a wearabledevice such as a smartwatch, a wireless hotspot, a femtocell, or abroadband router. Further, the user device in some cases may be embeddedin an electronic system such as the head unit of a vehicle or anadvanced driver assistance system (ADAS). Still further, the user devicecan operate as an internet-of-things (IoT) device or a mobile-internetdevice (MID). Depending on the type, the user device can include one ormore general-purpose processors, a computer-readable memory, a userinterface, one or more network interfaces, one or more sensors, etc.

Certain embodiments are described in this disclosure as including logicor a number of components or modules. Modules may can be softwaremodules (e.g., code, or machine-readable instructions stored onnon-transitory machine-readable medium) or hardware modules. A hardwaremodule is a tangible unit capable of performing certain operations andmay be configured or arranged in a certain manner. A hardware module caninclude dedicated circuitry or logic that is permanently configured(e.g., as a special-purpose processor, such as a field programmable gatearray (FPGA) or an application-specific integrated circuit (ASIC), adigital signal processor (DSP), etc.) to perform certain operations. Ahardware module may also have programmable logic or circuitry (e.g., asencompassed within a general-purpose processor or other programmableprocessor) that is temporarily configured by software to perform certainoperations. The decision to implement a hardware module in dedicated andpermanently configured circuitry, or in temporarily configured circuitry(e.g., configured by software) may be driven by cost and timeconsiderations.

When implemented in software, the techniques can be provided as part ofthe operating system, a library used by multiple applications, aparticular software application, etc. The software can be executed byone or more general-purpose processors or one or more special-purposeprocessors.

Upon reading this disclosure, those of skill in the art will appreciatestill additional and alternative structural and functional designs forhandling SCG failures in DC through the principles disclosed herein.Thus, while particular embodiments and applications have beenillustrated and described, it is to be understood that the disclosedembodiments are not limited to the precise construction and componentsdisclosed herein. Various modifications, changes and variations, whichwill be apparent to those of ordinary skill in the art, may be made inthe arrangement, operation and details of the method and apparatusdisclosed herein without departing from the spirit and scope defined inthe appended claims.

The following list of aspects reflects a variety of the embodimentsexplicitly contemplated by the present disclosure.

Aspect 1. A method in a base station operating as a master node (MN) forperforming a handover procedure comprises: configuring, by processinghardware, a UE that operates in dual connectivity (DC) with the MN and asecondary node (SN) with a data radio bearer (DRB) associated with asecondary cell group (SCG) of the SN; determining, by the processinghardware, that the DRB has been suspended; determining, by theprocessing hardware, that a handover procedure should be performed forthe UE; and initiating, by the processing hardware, the handoverprocedure when the DRB is suspended.

Aspect 2. The method of aspect 1, wherein determining that the DRB hasbeen suspended includes receiving an indication of failure of the SCGfrom the UE.

Aspect 3. The method of aspect 2, further comprising: in a firstinstance: receiving the indication of failure of the SCG in a dedicatedmessage for reporting SCG failure, associated with a protocol forcontrolling radio resources, and determining that the handover procedureis allowed; and in a second instance: receiving the indication offailure of the SCG in a message for reconfiguring a radio connection,associated with the protocol for controlling radio resources, anddetermining that the handover procedure is not allowed.

Aspect 4. The method of aspect 3, further comprising, in the secondinstance, subsequently to determining that the handover procedure is notallowed: transmitting to the UE a command to resume the suspended DRB;and in response to receiving an indication that the UE has resumed thesuspended DRB, determining that the handover procedure is not allowed.

Aspect 5. The method of aspect 1, further comprising: determiningwhether all of one or more DRBs with which the UE is configured aresuspended; and when not all of the one or more DRBs are suspended,determining that the handover procedure is allowed.

Aspect 6. The method of aspect 5, further comprising: when all of theone or more DRBs are suspended, determining whether at least one of theone or more DRBs is a master cell group (MCG) DRB or a split DRB; andwhen at least one of the one or more DRBs is an MCG DRB or a split DRB,determining that the handover procedure is not allowed.

Aspect 7. The method of aspect 5, further comprising: when all of theone or more DRBs are suspended, determining whether all of the one ormore DRBs are suspended due to a failure of the SCG; and when all of theone or more DRBs are suspended due to the failure of the SCG,determining that the handover procedure is allowed.

Aspect 8. The method of aspect 1, further comprising: receiving, by theprocessing hardware, signal measurement data for (i) a downlink carrierfrequency on which the UE currently operates and (ii) a downlink carrierfrequency to which the UE can be handed over; wherein determining thatthe handover procedure should be performed is based on the signalmeasurement data.

Aspect 9. The method of aspect 8, wherein the signal measurement data isincluded in an indication of failure of the SCG received from the UE.

Aspect 10. The method of aspect 8, wherein: determining that the DRB hasbeen suspended includes receiving an indication of failure of the SCGfrom the UE, and the signal measurement data is received in ameasurement report, prior to the indication of failure of the SCG.

Aspect 11. The method of aspect 8, wherein: determining that the DRB hasbeen suspended includes receiving an indication of failure of the SCGfrom the UE, and the signal measurement data is received in ameasurement report, subsequently to the indication of failure of theSCG.

Aspect 12. The method of aspect 1, wherein the base station is a firstbase station, and wherein initiating the handover procedure includes:transmitting a handover request to a second base station, receiving,from the second base station, an acknowledgement of the handoverrequest; and transmitting to the UE a command to hand over to the secondbase station.

Aspect 13. The method of aspect 1, wherein the base station is a firstbase station, and wherein initiating the handover procedure includes:transmitting to a core network (CN) an indication that a handover to asecond base station is required.

Aspect 14. The method of aspect 13, further comprising: receiving, fromthe CN, a handover command; and transmitting to the UE a command to handover to the second base station.

Aspect 15. The method of aspect 14, wherein the second base stationsupports a different radio access technology (RAT) than the first basestation.

Aspect 16. The method of aspect 1, wherein initiating the handoverprocedure includes: handing over the UE from a cell of the base stationto a second cell of the base station.

Aspect 17. The method of aspect 16, wherein handing over the UEincludes: transmitting to the UE a message associated with a protocolfor controlling radio resources, the message including a reconfigurationwith synchronization information element (IE).

Aspect 18. The method of aspect 1, wherein initiating the handoverprocedure includes handing over the UE from a first distributed unit(DU) of the base station to a second DU of the base station.

Aspect 19. The method of aspect 18, wherein handing over the UEincludes: transmitting to the UE a reconfiguration command associatedwith a protocol for controlling radio resources, the message including acell group configuration.

Aspect 20. A base station including processing hardware and configuredto implement a method according to any of the preceding aspects.

Aspect 21. A method in a UE operating in dual connectivity (DC) with amaster node (MN) and a secondary node (SN) for performing a handoverprocedure, the method comprising: obtaining, by processing hardware, adata radio bearer (DRB) associated with a secondary cell group (SCG) ofthe SN; determining, by the processing hardware, a failure of the SCG;in response to the failure, suspending, by the processing, the DRB;transmitting, by the processing hardware, an indication of the failureto the MN; and performing, by the processing hardware, a handoverprocedure when the DRB is suspended.

Aspect 22. The method of aspect 21, wherein transmitting the indicationof the failure to the MN includes transmitting a dedicated message forreporting SCG failure, associated with a protocol for controlling radioresources.

Aspect 23. The method of aspect 21, further comprising: generating, bythe processing hardware, signal measurement data in (i) a cell in whichthe UE currently operates and (ii) a cell to which the UE can hand over;and transmitting, by the processing hardware, the signal measurementdata to the MN.

Aspect 24. The method of aspect 23, wherein the cell to which the UE canhand over corresponds to a base station different from the MN.

Aspect 25. The method of aspect 23, wherein each of the cell in whichthe UE currently operates and the cell to which the UE can hand overcorrespond to the MN.

Aspect 26. The method of aspect 23, wherein: the MN is a disaggregatedMN, the cell in which the UE currently operates corresponds to a firstDU of the disaggregated MN, and the cell to which the UE can hand overcorresponds to a second DU of the disaggregated

MN.

Aspect 27. The method of aspect 21, further comprising: receiving an RRCReconfiguration message indicating a new cell to which the UE is to handover.

Aspect 28. A UE including processing hardware and configured toimplement a method according to any of aspects 21-27.

What is claimed is:
 1. A method in a base station operating as a masternode (MN) for performing a handover procedure, the method comprising:configuring, by processing hardware, a UE that operates in dualconnectivity (DC) with the MN and a secondary node (SN) with a dataradio bearer (DRB) associated with a secondary cell group (SCG) of theSN; determining, by the processing hardware, that the DRB has beensuspended; determining, by the processing hardware, that a handoverprocedure should be performed for the UE; and initiating, by theprocessing hardware, the handover procedure when the DRB is suspended.2. The method of claim 1, wherein determining that the DRB has beensuspended includes receiving an indication of failure of the SCG fromthe UE.
 3. The method of claim 2, further comprising: in a firstinstance: receiving the indication of failure of the SCG in a dedicatedmessage for reporting SCG failure, associated with a protocol forcontrolling radio resources, and determining that the handover procedureis allowed; and in a second instance: receiving the indication offailure of the SCG in a message for reconfiguring a radio connection,associated with the protocol for controlling radio resources, anddetermining that the handover procedure is not allowed.
 4. The method ofclaim 3, further comprising, in the second instance, subsequently todetermining that the handover procedure is not allowed: transmitting tothe UE a command to resume the suspended DRB; and in response toreceiving an indication that the UE has resumed the suspended DRB,determining that the handover procedure is not allowed.
 5. The method ofclaim 1, further comprising: determining whether all of one or more DRBswith which the UE is configured are suspended; and when not all of theone or more DRBs are suspended, determining that the handover procedureis allowed.
 6. The method of claim 1, further comprising: receiving, bythe processing hardware, signal measurement data for (i) a downlinkcarrier frequency on which the UE currently operates and (ii) a downlinkcarrier frequency to which the UE can be handed over; whereindetermining that the handover procedure should be performed is based onthe signal measurement data.
 7. The method of claim 1, wherein the basestation is a first base station, and wherein initiating the handoverprocedure includes: transmitting a handover request to a second basestation, receiving, from the second base station, an acknowledgement ofthe handover request; and transmitting to the UE a command to hand overto the second base station.
 8. The method of claim 1, wherein the basestation is a first base station, and wherein initiating the handoverprocedure includes: transmitting to a core network (CN) an indicationthat a handover to a second base station is required.
 9. The method ofclaim 1, wherein initiating the handover procedure includes: handingover the UE from a cell of the base station to a second cell of the basestation.
 10. The method of claim 1, wherein initiating the handoverprocedure includes handing over the UE from a first distributed unit(DU) of the base station to a second DU of the base station.
 11. Themethod of claim 10, wherein handing over the UE includes: transmittingto the UE a reconfiguration command associated with a protocol forcontrolling radio resources, the message including a cell groupconfiguration.
 12. A base station including processing hardware andconfigured to; configure a UE that operates in dual connectivity (DC)with the MN and a secondary node (SN) with a data radio bearer (DRB)associated with a secondary cell group (SCG) of the SN; determine thatthe DRB has been suspended; determine that a handover procedure shouldbe performed for the UE; and initiate the handover procedure when theDRB is suspended.
 13. A method in a UE operating in dual connectivity(DC) with a master node (MN) and a secondary node (SN) for performing ahandover procedure, the method comprising: obtaining, by processinghardware, a data radio bearer (DRB) associated with a secondary cellgroup (SCG) of the SN; determining, by the processing hardware, afailure of the SCG; in response to the failure, suspending, by theprocessing, the DRB; transmitting, by the processing hardware, anindication of the failure to the MN; and performing, by the processinghardware, a handover procedure when the DRB is suspended.
 14. The methodof claim 13, wherein transmitting the indication of the failure to theMN includes transmitting a dedicated message for reporting SCG failure,associated with a protocol for controlling radio resources.
 15. Themethod of claim 13, further comprising: generating, by the processinghardware, signal measurement data in (i) a cell in which the UEcurrently operates and (ii) a cell to which the UE can hand over; andtransmitting, by the processing hardware, the signal measurement data tothe MN.
 16. The base station of claim 12, wherein to determine that theDRB has been suspended, the processing hardware is configured to receivean indication of failure of the SCG from the UE.
 17. The base station ofclaim 16, wherein the processing hardware is further configured to: in afirst instance: (i) receive the indication of failure of the SCG in adedicated message for reporting SCG failure, associated with a protocolfor controlling radio resources, and (ii) determine that the handoverprocedure is allowed; and in a second instance: (i) receive theindication of failure of the SCG in a message for reconfiguring a radioconnection, associated with the protocol for controlling radioresources, and (ii) determine that the handover procedure is notallowed.
 18. The base station of claim 17, wherein the processinghardware is further configured to: in the second instance, subsequentlyto determining that the handover procedure is not allowed: transmit tothe UE a command to resume the suspended DRB; and in response toreceiving an indication that the UE has resumed the suspended DRB,determine that the handover procedure is not allowed.
 19. The basestation of claim 12, wherein the processing hardware is furtherconfigured to: determine whether all of one or more DRBs with which theUE is configured are suspended; and when not all of the one or more DRBsare suspended, determine that the handover procedure is allowed.
 20. Thebase station of claim 12, wherein the processing hardware is furtherconfigured to: receive signal measurement data for (i) a downlinkcarrier frequency on which the UE currently operates and (ii) a downlinkcarrier frequency to which the UE can be handed over; whereindetermining that the handover procedure should be performed is based onthe signal measurement data.